Camshaft adjusters are used in internal combustion engines to vary the timing of the combustion chamber valves. Adapting the timing to the current load reduces consumption and emissions. A camshaft adjuster is generally secured for conjoint rotation on a camshaft of the internal combustion engine and is in drive connection with a crankshaft. This drive connection can be implemented as a belt, chain or gear drive, for example, by means of a hydraulic phase adjustment device of the camshaft adjuster where it is possible to vary a phase relation between the crankshaft and the camshaft in a specific way by supplying or discharging pressure medium.
One widely used type of camshaft adjuster is the vane cell adjuster. Vane cell adjusters have a stator, a rotor and a drive wheel. The rotor is generally connected to the camshaft for conjoint rotation. The stator and the drive wheel are likewise connected to each other with the rotor being coaxial with the stator and within the stator. With their vanes, the rotor and the stator define oppositely acting oil chambers which can be supplied with oil pressure and enable a relative movement between the stator and the rotor. Moreover, the vane cell adjusters have various sealing covers. The assembly, which comprises the stator, the drive wheel and the sealing covers is formed by means of a plurality of screwed joints.
A camshaft adjuster is known from DE 195 29 277 A1, for example. The camshaft adjuster has a drive output element which is arranged so as to be rotatable relative to a drive input element. The drive input element is in drive connection with the crankshaft, and the drive output element is connected to the camshaft for conjoint rotation. The drive output element and the drive input element delimit a pressure space, which is divided into two oppositely acting pressure chambers by means of an axially movable piston. The piston is moved within the pressure space by supplying pressure medium to or discharging pressure medium from the pressure chambers. The piston has helical toothing, which meshes with helical toothing on the camshaft. Through one axial movement of the piston, it is thus possible to bring about a specific rotation of the camshaft relative to the crankshaft.
Moreover, a pressure accumulator is provided, which is arranged in a crankcase or a cylinder head of the internal combustion engine. During normal operation of the internal combustion engine, the pressure accumulator is filled with pressure medium, generally engine oil, by a pressure medium pump of the internal combustion engine. If the system pressure supplied by the pressure medium pump falls below a value required for functionally reliable operation of the camshaft adjuster, the pressure accumulator can be discharged into the pressure medium circuit of the internal combustion engine. It is thus possible to compensate for short-term undershooting of the minimum pressure within the pressure medium system and to increase the volume flow.
Another camshaft adjuster is known from EP 0 806 550 A1. This camshaft adjuster is in the form of a vane wheel, which has pressure medium distributor that is likewise assisted by a pressure accumulator. In this embodiment, the pressure accumulator is supposed to urge the camshaft adjuster into a phase position in which the internal combustion engine can be reliably restarted after having been switched off.
It is furthermore possible to prevent a dip in the speed of adjustment during the adjusting process. At the beginning of a phase adjustment operation, a certain quantity of pressure medium is taken from the pressure medium system of the internal combustion engine. As a result, the system pressure falls to a lower level. The system pressure present before the adjustment is not fully available for phase adjustment. The speed of phase adjustment and hence the performance of the internal combustion engine as a whole thus falls. If the pressure accumulator has been filled, this pressure drop is compensated by said accumulator and the speed of adjustment is maintained at a high level.